Fiber optic mechanical/thermal tuning and isolating device

ABSTRACT

A tunable fiber optic component providing environmental isolation, thermal tuning, and mechanical tuning and a method of tuning a fiber optic component using application of substantially simultaneous varying of temperature and mechanical strain is disclosed. A method of using a tunable fiber optic component, for example, a distributed feedback fiber laser, to compensate variations in an optical system, and a method of making a tunable fiber optic component are also disclosed.

The present application is a divisional of U.S. patent application Ser.No. 10/807,808, filed Mar. 23, 2004, which is a continuation-in-part ofU.S. patent application Ser. No. 10/017,162, filed Dec. 14, 2001, and acontinuation-in-part of U.S. patent application Ser. No. 10/383,909,filed Mar. 6, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to tuning of optical fiberstructures. More particularly, the present invention relates to tuning,protection, and environmental isolation of distributed feedback fiberlasers.

2. Related Art

Lasers have found use in a variety of applications, including displays,optical printing, optical recording, and communications. Distributedfeedback (DFB) fiber lasers, in particular, have proven useful in thoseapplications where the single-mode operation, narrow optical linewidth,and low noise performance of DFB fiber lasers are particularlyadvantageous. Tuning of DFB fiber laser optical frequency is generallyrequired to correct initial manufacturing tolerances, and various fixedschemes to accomplish initial tuning are known. In many applications,very stable laser optical frequency is required, however, andmaintaining long-term stability has proved challenging.

Controlled changes in temperature or controlled application ofmechanical strain to the fiber may be used to tune the optical frequencyof a DFB fiber laser. Changes in temperature and mechanical strainresult in changes to the period of gratings impressed into the fiberresulting in a change in the optical frequency of the DFB fiber laser.Thermal tuning by adjusting the temperature of a DFB fiber laser islimited to slow laser optical frequency changes, however, due to slowchange rates of the thermal mass of the fiber and associated temperaturecontrol mechanism.

Although mechanical tuning of a DFB fiber laser by the application ofmechanical strain can accomplish rapid changes in the laser opticalfrequency, such tuning is difficult. The DFB fiber laser manufacturingprocess weakens the fiber permitting excessive tension to break thefiber. Similarly, wide range mechanical tuning of the fiber can reducereliability due to the increased chance of fiber breakage. Although someof the problems associated with mechanical tuning can be avoided bylimiting the applied mechanical strain to compression loading ratherthan tension, application of compression to a fiber is difficult toachieve without buckling the fiber. Hence, complex mechanical structuresare required to prevent buckling when applying compression-onlymechanical tuning.

SUMMARY OF THE INVENTION

It has been recognized that it would be advantageous to develop atechnique to provide wide range and rapid tuning of the opticalfrequency of a fiber optic component simultaneously. Furthermore, it hasbeen recognized that it would be advantageous to provide tuning of theoptical frequency of a fiber optic component while also providingprotection of the fiber optic component and isolation of the fiber opticcomponent from acoustic vibration and mechanical shock. Furthermore, itis desirable to accomplish these objectives using a less complexstructure than prior art mechanically tuned lasers.

The invention provides a fiber optic component tuning and isolatingsupport device. The support device includes a tensioning structure inthermal contact with a thermal control structure which includes athermally-conductive, acoustic-damping material and athermally-controlled surface. In another embodiment, the inventionprovides a fiber optic component tuning and isolating support devicewhich includes a thermally-conductive, acoustic-damping material inthermal contact with a thermally-controlled surface and configured toreceive a fiber optic component. In another embodiment, the inventionprovides a fiber optic component tuning and isolating device whichincludes a tensioning structure encased in an acoustic-damping material.

The invention also provides a method of substantially simultaneouslythermally and mechanically tuning a fiber optic component. Anotherembodiment of the invention also provides a method of compensatingvariations in an optical system by thermally and mechanically tuning theoptical frequency of a DFB fiber laser.

Finally, the invention also provides a method of making a tunable fiberoptic component by encasing a fiber optic component in a tensioningstructure and affixing the tensioning structure to athermally-controlled surface using a thermally-conductive,acoustic-damping material.

Additional features and advantages of the invention will be apparentfrom the detailed description which follows, taken in conjunction withthe accompanying drawings, which together illustrate, by way of example,features of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a simultaneously thermally andmechanically tunable fiber optic component with integrated physicalprotection and environmental isolation in accordance with an embodimentof the present invention;

FIG. 2 is a cross sectional view of alternate embodiments of thetensioning structure of FIG. 1;

FIG. 3 is a perspective view of alternate embodiments of the tensioningstructure of FIG. 1;

FIG. 4 is a perspective view of a simultaneously thermally andmechanically tunable fiber optic component with integrated physicalprotection and environmental isolation in accordance with an embodimentof the present invention;

FIG. 5 is a perspective view of an alternate embodiment of a tensioningstructure;

FIG. 6 is a cross sectional view of the tensioning structure of FIG. 5;

FIG. 7 is a perspective view of a simultaneously thermally andmechanically tunable fiber optic component with integrated physicalprotection and environmental isolation in accordance with an embodimentof the present invention;

FIG. 8 is a cross sectional view of the device of FIG. 7; and

FIG. 9 is an illustration of the performance of a fiber optic lasertuner in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

Reference will now be made to the exemplary embodiments illustrated inthe drawing, and specific language will be used herein to describe thesame. It will nevertheless be understood that no limitation of the scopeof the invention is thereby intended. Alterations and furthermodifications of the inventive features illustrated herein, andadditional applications of the principles of the inventions asillustrated herein, which would occur to one skilled in the relevant artand having possession of this disclosure, are to be considered withinthe scope of the invention.

A system for simultaneous thermal tuning and mechanical tuning of afiber optic component with integrated physical protection andenvironmental isolation, in accordance with an embodiment of the presentinvention, is illustrated in FIG. 1 and indicated generally at 100. Thefiber optic component 12, for example, a DFB fiber laser, is attached toa tensioning structure 14. The tensioning structure is fabricated of anelectrically-active, mechanically-responsive material so that the lengthof the tensioning structure may be changed by electrical activation.

In accordance with one aspect of the present invention, the tensioningstructure 14 may be fabricated of a piezoelectric material 32 shaped inthe form of a tube. As illustrated in FIGS. 2A, 2B, 2C, 2D, and 2E,electrodes may be applied to the inner 22 and outer 24 surfaces of thetensioning structure 14 along the tube's length, so that the tube ispoled radially through its wall. The inner and outer electrodes areconnected via leads 28 to a voltage source whereby mechanical tuning ofthe optical frequency of the laser may be accomplished by electricalactivation of the piezoelectric material. The electrical activationcauses the tensioning structure to vary in length in the longitudinaldirection 26 according to the applied electrical signal. This in turnapplies varying tension to the fiber optic component 12, causing achange in the length and optical frequency of the fiber optic component.For example, if the fiber optic component is a DFB fiber laser, theresonant frequency of the laser will be changed; if a Bragg grating thegrating period will be changed.

This mechanical tuning using the tensioning structure 14 can providesmall optical frequency changes of a DFB fiber laser up to a few GHz.The rate at which frequency changes may be tuned is quite rapid: tuningrates of up to several hundred kHz may be obtained, depending primarilyon the mechanical frequency response of the tensioning structure, as isdiscussed further below.

Further detail of the attachment of the fiber optic component 12 andelectrical activation of the tensioning structure 14 is shown in thealternate embodiments illustrated in cross sectional views FIG. 2Athrough FIG. 2E. The fiber optic component may be attached to the endsof the tensioning structure using glue or clamps. For example, asillustrated in FIG. 2A, caps 34 may be installed at the ends of thetensioning structure and the fiber optic component glued to the caps.The caps may also used to provide electrical contact to the electrodes22, 24. For example, as illustrated in FIG. 2B, contacting caps 36, 38,fabricated of an electrically conductive material, may be used to makeelectrical contact to the inner and outer electrodes.

Alternately, as illustrated in FIG. 2C, the outer electrode can be splitinto two portions 24 a, 24 b and the inner electrode 22 may beelectrically connected through end plating 40 to connect the innerelectrode to part of the outer electrode 24 a. Two outer contacting caps38 make electrical contact to the outer electrode 24 b and to the innerelectrode 22 via end plating 40 and outer electrode part 24 a. Thisarrangement permits the use of symmetric end caps.

Optionally, the area within the bore 41 of the tensioning structure 14may be filled with a thermally-conductive, acoustic-damping material,such as oil 42. This will further enhance the acoustic isolation of thefiber optic component.

Various other arrangements of the electrical connection to theelectrodes are also possible. As illustrated in FIG. 2D, connection tothe inner electrode 22 may be accomplished by including a wire 62 withthe fiber optic component 12 inside the tensioning structure 14.Electrical contact between the wire and the inner electrode is ensuredby using a conductive epoxy 64, or similar, to secure the wire and fiberoptic component within the bore 41 of the tensioning structure. Theconductive epoxy also serves to bond the fiber optic component to thetensioning structure. Alternately, as illustrated in FIG. 2E, the fiberoptic component may include a conductive jacket 66 which is useddirectly to connect to the inner electrode, again using a conductiveepoxy, or similar material.

It is desirable that the tensioning structure extends longitudinally tocompletely encompass the active region of a DFB fiber laser to ensurethat uniform stress is applied to the gratings of the DFB fiber laserand to fully protect the DFB fiber laser. If the tensioning structure isdesigned to enclose only a portion of the active region of a DFB fiberlaser, this may result in poor efficiency or even bimodal output fromthe laser if stretched and unstretched portions of the DFB fiber laserhave different grating periods.

The tensioning structure 14 may be fabricated entirely of apiezoelectric or other similar material that provides electricallyactivated mechanical translation. Alternately, as shown in FIG. 3A, thetensioning structure may be fabricated from a rigid material 30, such asfused silica or alumina, in the center portion and a piezoelectricmaterial 32 at the ends. Preferably, the fiber optic component isattached only to the piezoelectric material at the ends of such acomposite tensioning structure to apply uniform mechanical strain to thelongitudinal dimension of the fiber optic component. A compositestructure as just described may, however, require higher appliedvoltages to the piezoelectric material to achieve the same longitudinaltranslation of the tensioning structure as the preferred embodiment. Asanother alternative, as shown in FIG. 3B, the tensioning structure maybe fabricated of alternating sections of rigid 30 and piezoelectricmaterial 32.

In accordance with another aspect of the present invention, thermaltuning of the optical frequency may be concurrently accomplished byvarying the temperature of the fiber optic component with a thermalcontrol structure. As illustrated in FIG. 1, the thermal controlstructure may be implemented using a thermally-controlled surface 18thermally connected to the fiber optic component via athermally-conductive, acoustic-damping material 16. Preferably, thethermally-conductive, acoustic-damping material is the sole means ofattaching the tensioning structure to the thermally-controlled surfaceto ensure isolation of the fiber optic component and tensioningstructure from acoustic vibration and mechanical shock. Variousthermally-conductive, acoustic-damping materials are known in the art.Various techniques of creating a thermally-controlled surface are knownin the art, including resistive heaters, thermoelectric coolers, heatpumps, or heat exchangers.

Thermal tuning using the thermally-controlled surface can provide largeoptical frequency changes up to several 100 GHz, complementing thenarrower tuning range provided by mechanical tuning using the tensioningstructure. The tuning rate of the thermally-controlled surface 18 issmaller than the mechanical tuning rate, allowing changing the opticalfrequency of the laser at tuning rates of up to about 5 Hz. Accordingly,the present invention provides both coarse tuning and fine tuning withinthe same structure.

The invention also solves problems inherent in trying to providephysical protection and environmental isolation of a tunable laser. Thetensioning structure 14 provides physical protection to fragile fiberoptic components. The tensioning structure may also provide the abilityto mechanically tune the laser without requiring hard mounting of thefiber optic component to any other structure than the tensioningstructure. The tensioning structure, containing the DFB fiber laser 12,may thus be mechanically isolated using thermally-conductive,acoustic-damping material 16. The acoustic-damping property isolates theDFB fiber laser from acoustic vibration and mechanical shock in thesurrounding environment. The thermally-conductive property thermallycouples the DFB fiber laser 12 to the thermally-controlled surface 18,ensuring that the DFB fiber laser 12 maintains substantially the sametemperature as the thermally-controlled surface 18 and thus isolatingthe DFB fiber laser 12 from ambient environmental temperature changes.If temperature control is not required, an acoustic-damping material maybe used instead of the thermally-conductive, acoustic-damping material.Conversely, if isolation from shock and vibration is not required, athermally-conductive material may be used instead of thethermally-conductive, acoustic-damping material.

Although the DFB fiber laser 12 may respond to pressure changes in theenvironment, these changes are slow and can be compensated by thethermal tuning. Hence, environmental isolation from ambient temperatureand mechanical shock and vibration is provided.

An alternate embodiment of a system for simultaneous thermal tuning andmechanical tuning of a fiber optic component with integrated physicalprotection and environmental isolation is illustrated in FIG. 4 andindicated generally at 200. The fiber optic component 12 is placedwithin a tensioning structure 14, which is placed within athermally-conductive, acoustic-damping material 16. Also placed withinthe thermally-conductive, acoustic-damping material 16 is a heaterelement 34. The heater element 34 may be connected by leads 36 to avoltage source which is used to control the temperature of thethermally-conductive, acoustic-damping material. The fiber opticcomponent is in thermal contact with the heater element through thethermally-conductive, acoustic-damping material, hence tuning of theoptical frequency of the DFB fiber laser 12 may be accomplished byelectrical control of the heater element 34. A second set of leads 28connected to the tensioning structure may be used to control thetensioning structure as previously discussed to accomplish mechanicaltuning of the DFB fiber laser.

Alternately, in accordance with another aspect of the present invention,the tensioning structure may be implemented as an open structure, givingup some physical protection of the fiber optic component for enhancedthermal coupling. For example, as illustrated in FIG. 5, and indicatedgenerally at 300, a tensioning structure main body 50 is fabricated ofan electrically activated, mechanically-responsive material, forexample, a piezoelectric material, with openings 52. Optionally, theinterior of the main body and openings may be filled with athermally-conductive, acoustic-damping material 16 as illustrated in thecross section view of FIG. 6. The thermally-conductive, acoustic-dampingmaterial further enhances the environmental isolation of the fiber opticcomponent 12 by improving thermal coupling to the thermally-controlledsurface 18.

In accordance with yet another aspect of the present invention, thetensioning structure may be implemented as a mounting plate. Forexample, as illustrated in FIG. 7, and indicated generally at 400, atensioning structure main body 60 is fabricated of an electricallyactivated, mechanically-responsive material. Clamps 62, affixed to themain body hold the fiber optic component 12 at each end of the main body60. The main body of the tensioning structure may be fabricated of apiezoelectric material, or a combination of piezoelectric and rigidmaterial as previously described. FIG. 8 illustrates a cross sectionview of the main body, and shows optional thermally-conductive,acoustic-damping material 16 used to thermally attach an optionalthermally-controlled surface 18. If thermal tuning is not required, anacoustic-damping material may be used instead of thethermally-conductive, acoustic-damping material. If mechanical tuning isnot required, the tensioning structure may be omitted. Finally, ifmechanical isolation is not required, a thermally-conductive materialmay be used instead of the thermally-conductive, acoustic-dampingmaterial.

An important aspect of the present invention is the length of the fiberoptic component. For example, a DFB fiber laser will exhibit amechanical natural frequency that depends on its length. Longer lasersexhibit a lower mechanical natural frequency; shorter lasers exhibit ahigher mechanical natural frequency. If the mechanical natural frequencyof the laser is too low, the mechanical tuning of the laser may performpoorly. Additionally, if the mechanical natural frequency of the laseris below about 20 kHz, the laser may be excessively responsive toacoustic vibration and mechanical shock, and it may prove difficult tomaintain a stable optical frequency.

On the other hand, the optical gain and optical linewidth of the laseralso depend on the length of the laser. Longer lasers provide greateroptical gain and narrower optical linewidth; shorter lasers provide lessoptical gain and wider optical linewidth. Depending on the particularapplication, an optimal length may therefore exist which balances adesire for a relatively high mechanical natural frequency with a desirefor relatively narrow optical linewidth. A DFB fiber laser length ofabout 15 mm has proven advantageous, although lengths of about 10 mm to50 mm may also prove useful, depending upon the particular application.

In accordance with another aspect of the current invention, thermaltuning using the thermally-controlled surface 18 may be accomplishedsimultaneously with mechanical tuning using the tensioning structure 14.Mechanical tuning can provide small optical frequency changes at highrates; conversely, thermal tuning can provide large optical frequencychanges at low rates. The combination of thermal tuning and mechanicaltuning thus provides a wide optical frequency tuning range withoutencountering the problem of reduced reliability, since the mechanicalstrain on the laser is limited to a small range. Large optical frequencychanges are accomplished using the thermal tuning, avoiding the need tooverstress the fiber. The invention also avoids the limited tuning speedthat thermal tuning alone provides by including simultaneous mechanicaltuning. By combining both types of tuning in a single structure, theinvention provides a more versatile fiber optic tuning structure usingfewer parts. Furthermore, improved isolation from environmental effects,such as acoustic vibration, mechanical shock, and temperature variationsis provided.

The simultaneous wide optical frequency range and rapid tuning rate ofDFB fiber laser optical frequency provided by the present invention maybe used to improve performance of optical systems. For example, whenusing resonant optical cavity injection, such as that disclosed byco-pending U.S. patent application Ser. No. 10/017,162, hereinincorporated by reference, it is necessary that the laser opticalfrequency and cavity optical resonant frequency be precisely matched toobtain efficient operation. Both laser optical frequency and cavityoptical resonant frequency can, however, be affected by a variety offactors, including temperature changes, mechanical drift and creep,mechanical shock, and acoustic vibration. This matching can beaccomplished with the present invention, for example, by tuning thelaser optical frequency to match the resonance of the cavity. Thermaltuning may adjust the DFB fiber laser optical frequency to match slowchanges in the resonant frequency of the cavity optical resonantfrequency caused by, for example, temperature changes, pressure changes,mechanical drift, or mechanical creep. Rapid mechanical tuning mayadjust the DFB fiber laser optical frequency to match the rapid changesin cavity resonant frequency caused by, for example, acoustic vibration,mechanical vibration, or mechanical shock. Furthermore, thermal tuningmay also be used to compensate any slow drift in the DFB fiber laseroptical frequency caused by, for example, mechanical creep or ambientpressure changes.

For example, FIG. 9 illustrates the performance of a fiber optic lasertuner in accordance with an embodiment of the present invention. Bothmechanical and thermal tuning of the laser were performed using a closedloop feedback system as described in U.S. patent application Ser. No.10/017,162. The mechanical tuning loop bandwidth was set on the order of100 kHz, and the thermal tuning loop bandwidth was set on the order of 1Hz, although various other loop bandwidths may advantageously be used.The upper curve of FIG. 9, labeled A, shows the piezoelectric activationvoltage, and the lower curve, labeled B, shows the thermal drivevoltage. At time T₀, the system was initially frequency locked, and themechanical loop piezoelectric activation voltage is approximately atcenter range. From time T₁ to T₂, the system is disturbed byperturbation, which is rapidly compensated by changes of thepiezoelectric activation voltage, due to the relatively wide bandwidthof the mechanical tuning loop. From time T₁ to T₃, the thermal tuningloop slowly adjusts the thermal drive voltage to return thepiezoelectric activation voltage back to center of its range. At timeT₄, a smaller perturbation is compensated by the piezoelectricactivation voltage.

It is to be understood that the above-referenced arrangements areillustrative of the application for the principles of the presentinvention. Numerous modifications and alternative arrangements can bedevised without departing from the spirit and scope of the presentinvention while the present invention has been shown in the drawings anddescribed above in connection with the exemplary embodiments(s) of theinvention. It will be apparent to those of ordinary skill in the artthat numerous modifications can be made without departing from theprinciples and concepts of the invention as set forth in the claims.

1. A tuning and isolating support device for a tunable fiber opticcomponent, comprising: a) a fiber optic tensioning structure having atleast opposing ends configured to rigidly attach to and along a commonlongitudinal axis with the tunable fiber optic component and formed ofan electrically-active, mechanically-responsive material such thatchanges in dimension along the longitudinal axis arising from a firstvoltage applied to the tensioning structure yield correspondingdimensional changes in the fiber optic component to provide tunableadjustment of an optical frequency of the tunable fiber optic component;and b) a thermal control structure coupled to the tensioning structureto control temperature of the fiber optic component and provide changesin the optical frequency by adjustment of the temperature, said thermalcontrol structure including: (i) a thermally-conductive,acoustic-damping material configured to minimize environmental vibrationeffects on the fiber optic component; and (ii) a thermally-controlledsurface in thermal contact with the thermally-conductiveacoustic-damping material, wherein the thermally-controlled surfacevaries in temperature and controls the temperature of the fiber opticcomponent so that the fiber optic component optical frequency is tunedat least partially in response to a second applied voltage.
 2. Thetuning and isolating support device of claim 1, wherein the tensioningstructure is fabricated from a piezoelectric material.
 3. The tuning andisolating support device of claim 1, wherein the tensioning structurecomprises at least one section of a piezoelectric material and at leastone section of a rigid material.
 4. The tuning and isolating supportdevice of claim 1, wherein the tensioning structure further comprises alongitudinal bore configured to receive the tunable fiber opticcomponent.
 5. The tuning and isolating support device of claim 4,further comprising acoustic-damping material disposed within the bore.6. The tuning and isolating support device of claim 4, wherein thetensioning structure further comprises at least one radially disposedaperture connecting the bore to an outer surface of the tensioningstructure.
 7. The tuning and isolating support device of claim 4,further comprising: c) a first electrode located on a first surface ofthe fiber optic tensioning structure; and d) a second electrode locatedon a second surface of the fiber optic tensioning structure where thefirst and second electrodes are configured to apply the first voltage tothe fiber optic tensioning structure.
 8. The tuning and isolatingsupport device of claim 7, wherein the first electrode is located on aninner surface of the fiber optic tensioning structure and the secondelectrode is located on an outer surface of the fiber optic tensioningstructure so that the first voltage is applied radially to thetensioning structure.
 9. The tuning and isolating support device ofclaim 4, further comprising a plug positioned within the end of thetensioning structure and having a hole configured to receive and rigidlyattach to the fiber optic component.
 10. The tuning and isolatingsupport device of claim 1, further comprising the tunable fiber opticcomponent longitudinally mounted and rigidly attached to the tensioningstructure.
 11. The tuning and isolating support device of claim 10,wherein the tunable fiber optic component comprises a conductive jacketused to provide an electrical connection to the tensioning structure.12. The tuning and isolating support device of claim 10, wherein thefiber optic component is rigidly attached to the tensioning structurewith glue.
 13. The tuning and isolating support device of claim 10,wherein the fiber optic component is rigidly attached to the tensioningstructure with clamps.
 14. The tuning and isolating support device ofclaim 10, wherein the fiber optic component is configured as adistributed feedback fiber laser mounted with an active region of thedistributed feedback fiber laser positioned substantially between theends of the tensioning structure.
 15. The tuning and isolating supportdevice of claim 14 wherein a length of the distributed feedback fiberlaser is optimally selected to provide a particular mechanical naturalfrequency and optical linewidth.
 16. The tuning and isolating supportdevice of claim 1, wherein the thermally-controlled surface comprises aresistive heater wire embedded within the thermally-conductive,acoustic-damping material.
 17. The tuning and isolating support deviceof claim 1, wherein the thermally-controlled surface is selected fromthe group consisting of a thermoelectric cooler, a heat pump, and a heatexchanger.
 18. The tuning and isolating support device of claim 1wherein the tensioning structure has an operable bandwidth relative tothe first voltage of about 0 to 500 kHz.
 19. The tuning and isolatingsupport device of claim 1 wherein the thermal control structure has anoperable bandwidth relative to the second voltage of about 0 to 5 Hz.